1. Field of the Invention
The present invention relates to a method of manufacturing a tunnel magnetoresistive element utilizing the tunnel magnetoresistive effect, a method of manufacturing a thin-film magnetic head incorporating the tunnel magnetoresistive element, and a method of manufacturing a memory element incorporating the tunnel magnetoresistive element.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as recording density of hard disk drives has increased. Such thin-film magnetic heads include composite thin-film magnetic heads that have been widely used. A composite head is made of a layered structure including a recording head having an induction-type electromagnetic transducer for writing and a reproducing head having a magnetoresistive element for reading.
Reproducing heads that exhibit high sensitivity and produce high outputs have been required. In response to such demands, attention has been focused on tunnel magnetoresistive elements (that may be hereinafter called TMR elements) that detect a magnetic field through the use of the tunnel magnetoresistive effect.
The TMR element has a structure in which a lower magnetic layer, a tunnel barrier layer and an upper magnetic layer are stacked. Each of the lower and upper magnetic layers include a ferromagnetic substance. In general, the magnetic layer closer to the substrate is called the lower magnetic layer and the magnetic layer farther from the substrate is called the upper magnetic layer. Therefore, the terms xe2x80x98upperxe2x80x99 and xe2x80x98lowerxe2x80x99 of the upper and lower magnetic layers do not always correspond to the position in the arrangement of an actual TMR element.
The tunnel barrier layer is a layer made of a thin nonmagnetic insulating film through which electrons are capable of passing while maintaining spins thereof by means of the tunnel effect, that is, through which a tunnel current is allowed to pass. The tunnel magnetoresistive effect is a phenomenon that, when a current is fed to a pair of magnetic layers sandwiching the tunnel barrier layer, a tunnel current passing through the tunnel barrier layer changes, depending on the relative angle between magnetizations of the two magnetic layers. If the relative angle between magnetizations of the magnetic layers is small, the tunneling rate is high. As a result, the resistance to the current passing across the magnetic layers is reduced. If the relative angle between magnetizations of the magnetic layers is large, the tunneling rate is low. The resistance to the current passing across the magnetic layers is therefore increased.
A prior-art TMR element is formed by stacking the lower magnetic layer, the tunnel barrier layer and the upper magnetic layer in this order on the substrate. The tunnel barrier layer is formed by, for example, making a layer of a nonmagnetic substance, which is represented by aluminum (Al), on the lower magnetic layer, and oxidizing this layer through a method such as natural oxidation or plasma oxidation.
To apply a TMR element to the head of a hard disk drive, it is necessary to reduce the resistance of the TMR element in terms of not only performance characteristics but also mass-productivity. It is known that a reduction in thickness of the tunnel barrier layer achieves a reduction in resistance of the TMR element.
Reference is now made to FIG. 16 to describe an experiment performed to determine the relationship among the thickness of the tunnel barrier layer, the resistance of the TMR element, and the rate of change in the resistance of the TMR element. Elements having the following structure were used for this experiment. Each of the elements had a lower electrode layer made up of three layers of a Ta layer having a thickness of 5 nm, a Cu layer having a thickness of 50 nm, and a Ta layer having a thickness of 5 nm. On the lower electrode layer, the following layers were stacked one by one: a free layer made up of two layers of a NiFe layer having a thickness of 3 nm and a CoFe layer having a thickness of 3 nm; a tunnel barrier layer; a pinned layer made up of two layers of a CoFe layer having a thickness of 3 nm and a PtMn layer having a thickness of 17 nm; and an upper electrode layer made up of two layers of a Cu layer having a thickness of 50 nm and a Ta layer having a thickness of 5 nm. The tunnel barrier layer was formed by making an Al layer having a specific thickness through sputtering and oxidizing the Al layer in an oxygen atmosphere at 200 Torr (26664.4 Pa) for one hour. The area of the free layer, the tunnel barrier layer and the pinned layer joined to each other (hereinafter called the size of the TMR element) was 1 xcexcm by 1 xcexcm.
Immediately before the Al layer was formed, the center line average roughness Ra of the CoFe layer, which was the base layer of the Al layer, was 0.23 nm. The center line average roughness Ra indicates the flatness of the surface of the CoFe layer. Therefore, the CoFe layer surface was relatively flat. The Al layer was formed at a room temperature. It is estimated that the temperature of the substrate when the Al layer was formed was approximately 40 to 50xc2x0 C., due to the energy exerted during sputtering.
FIG. 16 shows the relationship among the tunnel barrier layer thickness, the resistance of the TMR element, and the maximum rate of change in the resistance of the TMR element (simply shown as resistance change rate in the table) obtained from the experiment performed under the above-mentioned conditions. The resistance change rate was obtained from an amount of change in the resistance of the TMR element when an external magnetic field was changed, the amount of change being divided by a minimum resistance value and being indicated in percent.
As shown in FIG. 16, the resistance of the TMR element decreased while the maximum rate of change in the resistance increased, as the thickness of the tunnel barrier layer (Al layer) was reduced to 0.7 nm. The characteristics of the TMR element were thereby improved. However, if the tunnel barrier layer thickness was 0.6 nm or smaller, the maximum rate of change in the resistance was made too small and the characteristics of the TMR element were extremely reduced. This is because, if the Al layer to be the tunnel barrier layer was too thin, it was impossible that the Al layer had a continuous structure. As a result, a leakage current other than a tunnel current flew through the tunnel barrier layer. The minimum thickness of the Al layer capable of forming a continuous structure is hereinafter called a critical thickness. The critical thickness was 0.7 nm, according to the experiment result shown in FIG. 16.
To meet recording density of 40 gigabits per square inch or greater, it is expected that the size of a TMR element is required to be as small as or smaller than 0.4 xcexcm by 0.4 xcexcm. According to the experiment result shown in FIG. 16, when the tunnel barrier layer thickness was 0.7 nm, the resistance of the TMR element of 1 xcexcm by 1 xcexcm was 30.4 ohm. Therefore, when the critical thickness is about 0.7 nm, the resistance of the TMR element is 100 ohm or greater, if the size of the TMR element is as small as or smaller than 0.4 xcexcm by 0.4 xcexcm. This resistance is not small enough, regarding the demand for obtaining low-resistance TMR elements.
It is an object of the invention to provide methods of manufacturing a tunnel magnetoresistive element, a thin-film magnetic head and a memory element, for reducing the thickness of a tunnel barrier layer without reducing performance characteristics.
A method of the invention is provided for manufacturing a tunnel magnetoresistive element comprising a tunnel barrier layer and first and second magnetic layers sandwiching the tunnel barrier layer. The method includes the steps of: forming the first magnetic layer on a substrate; forming the tunnel barrier layer on the first magnetic layer; forming the second magnetic layer on the tunnel barrier layer; and cooling the substrate prior to or at the same time as the step of forming the tunnel barrier layer. The step of forming the tunnel barrier layer includes the step of forming a layer of a material used for making the tunnel barrier layer on the first magnetic layer while the substrate remains cooled.
A method of the invention is provided for manufacturing a thin-film magnetic head incorporating a tunnel magnetoresistive element comprising a tunnel barrier layer and first and second magnetic layers sandwiching the tunnel barrier layer. The method includes the above-described steps.
A method of the invention is provided for manufacturing a memory element incorporating a tunnel magnetoresistive element comprising a tunnel barrier layer and first and second magnetic layers sandwiching the tunnel barrier layer. The method includes the above-described steps.
According to the methods of manufacturing the tunnel magnetoresistive element, the thin-film magnetic head or the memory element of the invention, the layer of the material used for making the tunnel barrier layer is formed on the first magnetic layer while the substrate remains cooled. As a result, the layer thus made easily forms a continuous structure. It is thereby possible to further reduce the thickness of the tunnel barrier layer.
According to the methods of manufacturing the tunnel magnetoresistive element, the thin-film magnetic head or the memory element of the invention, the step of forming the tunnel barrier layer may include the steps of: forming a layer of a nonmagnetic metal material, which is the material used for making the tunnel barrier layer, on the first magnetic layer while the substrate remains cooled; and oxidizing the layer of the nonmagnetic metal material to form the tunnel barrier layer.
According to the methods of manufacturing the tunnel magnetoresistive element, the thin-film magnetic head or the memory element of the invention, the substrate may be cooled only in the step of forming the tunnel barrier layer, among the steps of forming the first magnetic layer, forming the tunnel barrier layer, and forming the second magnetic layer.
According to the methods of manufacturing the tunnel magnetoresistive element, the thin-film magnetic head or the memory element of the invention, the step of forming the first magnetic layer may include flattening of a surface to be a base of the tunnel barrier layer.
According to the methods of manufacturing the tunnel magnetoresistive element, the thin-film magnetic head or the memory element of the invention, in the step of forming the layer of the material used for making the tunnel barrier layer, this layer may be formed at a rate lower than a rate at which the other layers are formed in the steps of forming the other layers.
Other and further objects, features and advantages of the invention will appear more fully from the following description.